Over the past few years, considerable improvements have been achieved in the design and performance of inductively coupled plasma torches, the so-called induction plasma torches. Induction plasma torches are currently used worldwide for a wide range of applications, ranging from laboratory R&D to industrial scale production of high purity, high added value materials.
Induction plasma torches have attracted increasing attention as a valuable tool for synthesis of materials and processing under high temperature plasma conditions. The basic concept behind the operation of induction plasma torches has been known for more than sixty years and has evolved steadily form a laboratory tool to an industrial, high power device.
FIG. 1 is a schematic illustration of the structure and operation of an example of induction plasma torch 100. The induction plasma torch 100 comprises a plasma confinement tube 102 which may, for example, be made of high-temperature-resistant and high thermal conductivity ceramic material. The plasma confinement tube 102 is surrounded by a coaxial, water-cooled induction coil 104 embedded in a coaxial, tubular torch body 106. A high frequency electrical current is supplied to the induction coil 104 through electric terminals 105. A gas distributor head (not shown) supplies a plasma gas 108 axially and centrally into an inner space of the plasma confinement tube 102 to produce a plasma 110. Variants may include injection of a sheath gas 112 flowing along the inner surface of the plasma confinement tube 102 to surround the plasma 110. A function of the sheath gas 112 it to provide some level of heat insulation between the plasma 110 and the inner surface of the plasma confinement tube 102. The induction plasma torch 100 may be used, in particular but not exclusively, to process powder material 114 injected centrally within the plasma confinement tube 102.
In operation, the high frequency electrical current flowing though the induction coil 104 creates within the plasma confinement tube 102 a generally axial high frequency magnetic field 120. The energy of this magnetic field 120 causes electrical breakdown of the plasma gas 108 present in the plasma confinement tube 102. Once electrical breakdown and plasma ignition is achieved, a tangential current is induced into the plasma gas in a region 122 within the plasma confinement tube 102 at the level where the induction coil 104 is located. This induced, tangential current is responsible for heating the plasma gas 108 in the plasma confinement tube 102 and sustaining the plasma gas discharge forming the plasma 110.
Numerous designs of induction plasma torches have been developed. Examples are described in the following patent publications: U.S. Pat. No. 5,200,595 (Apr. 6, 1993), U.S. Pat. No. 5,560,844 (Oct. 1, 1996), U.S. Pat. No. 6,693,253 B2 (Feb. 17, 2004), U.S. Pat. No. 6,919,527 B2 (Jul. 19, 2005) and US patent publication 2012/0261390 A1 (Oct. 18, 2012). The contents of all these references are incorporated by reference herein in their entirety.
Energy density in the plasma 110 is defined as the ratio of the energy coupled into the plasma 110 in region 122, to the volume of a discharge cavity as defined by the inner surface (i.e. boundary) of the plasma confinement tube 102 and the height of the induction coil 104. An increase of the energy density in the plasma 110 is manifested by an increase of the bulk specific enthalpy of the plasma, as well as by an increase of a corresponding average temperature of the plasma 110 at an exit 124 of the induction plasma torch 100. Unfortunately, this increase of the energy density is also accompanied by an increase in a heat flux to the inner surface of the plasma confinement tube 102, thereby causing an increase of the temperature of its inner surface and consequently the chance of tube failure.
To reduce the temperature of the inner surface of plasma confinement tube, a solution comprises the use of a high thermal conductivity ceramic material in the manufacture of the plasma confinement tube and the flow of a cooling fluid at high velocity in an annular channel surrounding the outer surface of the plasma confinement tube. However, despite the addition of these features, the maximum energy density of the plasma in an induction plasma torch is still limited by the maximum temperature that the high thermal conductivity ceramic material of the plasma confinement tube can withstand while keeping its structural integrity.
Another problem encountered when using induction plasma torches such as 100 in FIG. 1 is the creation of stray-arcing between (a) the plasma gas discharge 110 and (b) an exit nozzle (not shown in FIG. 1) of the induction plasma torch 100 and/or the body of a reactor (not shown in FIG. 1) on which the induction plasma torch 100 is mounted.
Therefore, there is a need for increasing the plasma energy density while, if not eliminating, substantially reducing stray-arcing in induction plasma torches.